Females of a Lake Victoria haplochromine cichlid can select the best sexual partners by choosing the males with the deepest red colouration and the largest territories, as the fish have the least parasites.

Females of the haplochromine cichlid Pundamilia nyererei are predominantly attracted by the red colouration of males and territory size, both of which are believed to be indicators of male quality or fitness.

An International team of fish biologists from the University of Leiden in the Netherlands, the University of Alcala in Madrid and the University of Bern in Switzerland, studied P. nyererei and found that the most colourful males with the largest territories were more attractive to females and had a lower number of parasites than duller males with smaller territories.

They believe that the results show that these female preferences select against heavily parasitised males via a process known as parasite-mediated sexual selection. The males with the deepest red colouration and the biggest territories were stronger fish and had fewer parasites, so the females selected these fish as mates over duller males which lacked the aggression to maintain a bigger territory.

The nuptial (or breeding) colouration of Pundamilia nyereri is subject to intraspecific sexual selection by female mate choice, and is also important in preventing the hybridisation of the species with the similar-looking Pundamilia pundamilia, which lives alongside it in the Lake.

Said the team: "P. nyererei male colouration is carotenoid based, illustrating the potential for honest signaling of individual quality. In a wild population, we found that variation in male colouration was not associated with variation in a set of strongly intercorrelated indicators of male dominance: male size, territory size and territory location.

"Instead, the 2 male characters that predominantly determine female choice, territory size and red coloration, may be independent predictors of male quality: males with bright red coloration and large territories had lower parasite infestation rates. As a result, female preferences tended to select against heavily parasitized males. Consistent with parasite-mediated sexual selection, males had higher and more variable parasite loads than females."